1. Field of the Invention
The present invention relates to a cardiac artifact removal technique, and more particularly, concerns a method and apparatus for removing cardiac artifact during the monitoring of a patient's respiration using impedance plethysmography when the patient has a regular, substantially periodic heartbeat.
2. Description of the Prior Art
Monitoring the respiration of a patient can be performed electrically by impedance plethysmography. Respiration characteristics of the patient are determined by measuring changes in chest impedance. Respiration produces variations in the shape and content of the patient's chest that are electrically detectable as impedance variations. Electrodes placed on the patient's chest allow these impedance variations to be monitored.
The respiration signal developed through the electrodes essentially is a measure of transchest impedance. In determining the patient's respiration, the impedance variation that is of most significance is a result of change in the volume of air in the lungs. However, inasmuch as the chest and its contents are affected by other activities, the monitored impedance will also be a function of these activities. For instance, any change of blood volume in the chest will cause a change in the impedance signal, as will any change in the chest shape or size such as caused by moving or attempting to breathe with the trachea blocked. Each of these latter activities contributes to the change of impedance. Therefore, the impedance signal which is intended to correlate closely with respiration rate also has these extraneous contributions which basically are undesirable artifacts with respect to pure respiration monitoring.
An abnormal patient movement may be easy to detect and can be readily discarded by the attendant who monitors the impedance changes. However, artifact is also introduced into the impedance signal by varying pleural blood volume, which is caused by the action of the heart in pumping blood during each heartbeat. Since this cardiac activity is ongoing constantly at the same time respiration is being monitored, this cardiac artifact is likely to be present in the signal most of the time. Accordingly, it would be most desirable to be able to eliminate this cardiac artifact due to normal heart activity from the impedance signal so that the signal is almost purely related to the respiration of the patient.
Various respiration monitors have been described in U.S. Pat. Nos. 3,536,062; 3,572,317; 3,802,419; and 3,976,052. However, none of these patented systems provides a technique for removal of cardiac artifact from the impedance signal.